U.S. patent application number 11/113694 was filed with the patent office on 2007-02-01 for detection of nuclear weapons and fissile material aboard cargo containerships.
Invention is credited to Shawn P. Gallagher, Richard C. Lanza.
Application Number | 20070023665 11/113694 |
Document ID | / |
Family ID | 36647905 |
Filed Date | 2007-02-01 |
United States Patent
Application |
20070023665 |
Kind Code |
A1 |
Gallagher; Shawn P. ; et
al. |
February 1, 2007 |
DETECTION OF NUCLEAR WEAPONS AND FISSILE MATERIAL ABOARD CARGO
CONTAINERSHIPS
Abstract
A radiation detection system that measures radiation (e.g.,
signature energy-level gamma radiation and neutrons) is employed
aboard a ship or other transport vehicle along with conventional
cargo to monitor for the presence of a fissile material, as would
be found in a nuclear weapon, or for the presence of other sources
of radiation. The detection system can be used over the course of
the cargo transport, thereby enabling finely tuned monitoring for
fissile material across distances of many meters extending through
surrounding cargo containers. Because the system can be utilized
during a ship's transport, a positive detection of fissile material
can be made and acted upon while the ship is still at sea far from
the destination port, where detonation of a nuclear weapon could
have catastrophic consequences.
Inventors: |
Gallagher; Shawn P.;
(Attleboro, MA) ; Lanza; Richard C.; (Brookline,
MA) |
Correspondence
Address: |
MINTZ, LEVIN, COHN, FERRIS, GLOVSKY;AND POPEO, P.C.
ONE FINANCIAL CENTER
BOSTON
MA
02111
US
|
Family ID: |
36647905 |
Appl. No.: |
11/113694 |
Filed: |
April 25, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60566257 |
Apr 29, 2004 |
|
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Current U.S.
Class: |
250/358.1 |
Current CPC
Class: |
G01T 3/00 20130101; G01T
1/167 20130101; G01V 5/0091 20130101 |
Class at
Publication: |
250/358.1 |
International
Class: |
G01T 1/00 20070101
G01T001/00 |
Claims
1. (canceled)
2. (canceled)
3. The method of claim 8, wherein the detected radiation further
includes neutrons.
4. The method of claim 3, wherein the gamma radiation is detected
with gamma detectors and the neutrons are detected with neutron
detectors that are distinct from the gamma detectors.
5. A method for detecting a radiation source aboard a cargo
containership comprising: loading a plurality of containers onto a
cargo containership, wherein at least one but fewer than all of the
containers is a detection container that contains a detection
system configured to detect radiation entering the detection
container from another container; recording detection of gamma
radiation by the detection system and determining if the detected
gamma radiation exceeds a threshold so as to indicate the presence
of the radiation source, the detection container also containing a
transmitter for transmitting a signal beyond the detection
container if the threshold is exceeded, wherein the spatial
distribution of gamma radiation entering the detection container is
imaged with an array of gamma detectors.
6. The method of claim 5, wherein the measurement of the spatial
distribution of the detected gamma radiation is evaluated to
determine whether the radiation comes from a concentrated
source.
7. The method of claim 6, wherein the detected gamma radiation
passes through a coded-aperture mask in the detection container
before being detected by a gamma detector in the array.
8. A method for detecting a radiation source aboard a cargo
containership comprising: loading a plurality of containers onto a
cargo containership, wherein at least one but fewer than all of the
containers is a detection container that contains a detection
system configured to detect radiation entering the detection
container from another container; and recording detection of
radiation by the detection system and determining if the detected
radiation exceeds a threshold so as to indicate the presence of the
radiation source, the detection container also containing a
transmitter for transmitting a signal beyond the detection
container if the threshold is exceeded, wherein gamma radiation at
2615 keV is specifically measured and evaluated to determine
whether the .sup.208T1 daughter product of .sup.232U is
present.
9. The method of claim 8, wherein gamma radiation at 1001 keV also
is specifically measured and evaluated to confirm whether a 2615
keV peak is from the decay of uranium or from other sources.
10. The method of claim 8, wherein gamma radiation at 911 keV also
is specifically measured and evaluated to confirm whether the 2615
keV peak is from the decay of uranium or from .sup.228Ac, a
daughter of .sup.232Th.
11. The method of claim 8, wherein the radiation is detected and
the data is transmitted while the ship is at sea away from
port.
12. The method of claim 11, wherein neutrons and gamma radiation
are detected aboard the cargo containership for a plurality of
days.
13. The method of claim 8, wherein at least one detection container
has substantially the same dimensions as the containers that are
not detection containers, and wherein the presence of the detection
system in that detection container is not visibly discernable from
outside the detection container.
14. A method for detecting a radiation source aboard a cargo
containership comprising: loading a plurality of containers onto a
cargo containership, wherein at least one but fewer than all of the
containers is a detection container that contains a detection
system configured to detect radiation entering the detection
container from another container; and recording detection of
radiation by the detection system and determining if the detected
radiation exceeds a threshold so as to indicate the presence of the
radiation source the detection container also containing a
transmitter for transmitting a signal beyond the detection
container if the threshold is exceeded, wherein the contents of at
least one detection containers essentially of components that are
part of, connected to, or in communication with the detection
system.
15. The method of claim 8, further comprising transmitting a signal
including data relating to detected radiation from the container to
a receiver outside the detection container.
16. The method of claim 15, further comprising transmitting a
signal from inside a dummy container aboard the ship, where the
dummy container does not contain a detection system.
17. The method of claim 8, wherein between 3 and 20 detection
containers are loaded onto the ship along with more than 20
containers that do not contain detector systems.
18. A method for detecting a radiation source aboard a cargo
containership comprising: loading a plurality of containers onto a
cargo containership, wherein at least one but fewer than all of the
containers is a detection container that contains a detection
system configured to detect radiation entering the detection
container from another container and an active interrogation system
that includes a source configured to emit photons or neutrons; and
recording detection of radiation by the detection system and
determining if the detected radiation exceeds a threshold so as to
indicate the presence of the radiation source, the detection
container also containing a transmitter for transmitting a signal
beyond the detection container if the threshold is exceeded.
19. The method of claim 18, further comprising using a time stamp
to track the timing of emissions from the active interrogation
system and to track the timing of radiation detection.
20. A method for detecting a radiation source aboard a cargo
containership comprising: loading a plurality of containers in the
form of car bodies onto a cargo containership, wherein at least one
but fewer than all of the containers is a detection container that
contains a detection system configured to detect radiation entering
the detection container from another container; and recording
detection of radiation by the detection system and determining if
the detected radiation exceeds a threshold so as to indicate the
presence of the radiation source, the detection container also
containing a transmitter for transmitting a signal beyond the
detection container if the threshold is exceeded.
21. A method for detecting a radiation source aboard a cargo
containership comprising: loading a plurality of containers onto a
cargo containership, wherein a plurality but fewer than all of the
containers are detection containers, each containing a detection
system configured to detect radiation entering the detection
container from another container; and wherein the detection systems
contained in distinct detection containers communicate with each
other; and recording detection of radiation by the detection system
and determining if the detected radiation exceeds a threshold so as
to indicate the presence of the radiation source, the detection
container also containing a transmitter for transmitting a signal
beyond the detection container if the threshold is exceeded.
22. The method of claim 21, wherein radiation detectors in the
detection systems are mounted and configured for pivoting so as to
face a suspected radiation source in response to a determination
that the radiation detected by a detection system has exceeded the
threshold.
23. (canceled)
24. A detection container comprising: a cargo container having: a
length of about 20 feet or about 40 feet; a width of about 8 feet;
and a height of about 8.5 or about 9.5 feet; and a detection system
including: at least one radiation detector coupled with a power
source, contained within the container and configured to detect
radiation from outside the container, wherein the detector is a
gamma-ray detector configured to detect .sup.232U decay series
gamma radiation; and a transmitter contained within the cargo
container, the transmitter being coupled with the radiation
detector for transmitting data from the radiation detector.
25. The detection container of claim 24, wherein the gamma-ray
detector is in the form of a detector array.
26. The detection container of claim 25, wherein the detector array
comprises a plurality of detector elements that are oriented in
different directions so as to enable imaging of gamma radiation
sources from any direction.
27. The detection container of claim 25, the detection system
further comprising a coded-aperture mask in front of the array.
28. The detection container of claim 24, wherein a neutron detector
is also coupled with the power source.
29-31. (canceled)
Description
RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/566,257, filed Apr. 29, 2004, the entire
teachings of which are incorporated herein by reference.
BACKGROUND
[0002] The end of the Cold War and the ascendance of transnational
terrorism have fundamentally changed the nature of the threat posed
by nuclear weapons to the United States (and to other nations).
What remains constant is the understanding that the detonation of a
single nuclear device on American soil would have immediate and
profound impacts, the scale and breadth of which are difficult to
comprehend. Given the openly professed desire of terrorist
organizations to obtain nuclear weapons and their demonstrated
willingness to sow widespread destruction, serious efforts are
clearly required to assess this nation's vulnerabilities and to
enhance our security posture. One potential class of
vulnerabilities are U.S. seaports where currently an average of
15,600 cargo containers arrive by ship every day, any one of which
could be used by an adversary to conceal fissile material or an
assembled nuclear device.
[0003] The currently prevailing model for addressing threats
associated with cargo-container-borne fissile material could be
characterized as a customs-based approach, where radiation
detection systems are integrated into the existing customs
infrastructure at ports. The system scans suspect containers as
they are unloaded from the ship or as they are subsequently loaded
onto a truck or train for inland transport. During this scanning
operation, the system utilizes either active or passive techniques
to detect nuclear signatures from fissile material in the
containers.
[0004] The passive scanning technique utilizes detectors to scan
for the normal radiation signal from fissile material, while the
active scanning technique sends a beam of neutrons or photons
towards the cargo causing a unique return signal from subsequent
fission events. Most systems have focused on the placement of
detector arrays in a wall on or near the docks. Some of the
ground-mounted arrays are situated such that semi-trucks or trains
pass through detector portals along their normal routes. Almost all
of these systems require the vehicle to stop for a few minutes so
that a statistically significant number of counts can be
collected.
[0005] The customs-based approach is a natural extension of
conventional strategies for finding and seizing incoming
contraband. Nuclear weapons, however, are unlike any other kind of
contraband in that their power is so great, and their effects so
far-reaching, that discovering them in port (or even allowing them
to enter port at all) cannot provide adequate protection from their
destructive reach.
SUMMARY
[0006] We present here an approach whereby systems for detecting
fissile material or other sources of radiation are deployed not in
fixed locations at ports, but instead enclosed within standard,
nondescript cargo containers to be carried aboard the
containerships themselves. To reap returns in sensitivity, stealth,
and most importantly, standoff, the ship-based approach of this
disclosure replaces the fixed-location, short-duration,
proximate-distance signature acquisition characteristic of the
customs-based approach with mobile containerized detections units
that counterbalance weaker signals from more-distant sources by
providing dramatically increased count times.
[0007] The detection system measures gamma radiation at signature
energies as well as measuring neutron emissions and compares those
readings with known background values to evaluate whether a nuclear
weapon or other source of radiation (e.g. a dirty bomb) is aboard
the ship. For a plutonium-based weapon, detection can be directed
to neutrons from the spontaneous fission of .sup.240Pu; and for a
uranium-based weapon, detection can focus on signals from
impurities, such as .sup.232U, preferentially accumulated during
the enrichment process. For example, the detection system can be
used to detect 1001-keV and 2615-keV gamma radiation, which is a
signature of uranium decay processes.
[0008] The gamma detector can be provided in the form of an array
and used to measure the spatial distribution of detected gamma
radiation (i.e, to image the radiation) so as to be able to
evaluate whether the source is concentrated. Further, a plurality
of arrays can be provided to enable the detector to image radiation
coming from any direction. Further still, a coded-aperture mask can
be mounted in front (or on opposite sides) of each detector array
to improve the imaging capability of the detector.
[0009] Further still, the passage of radiation through cargo
containers can be simulated, e.g., using Monte Carlo N-Particle
Transport Code (MCNP), wherein density values are assigned to
three-dimensional pixels in a virtual cargo container as a
probability-weighted function of known cargo density percentages in
containers (based, e.g., on logs of cargo-content volumes shipped
via existing routes). The passage of photons through the pixels
along a path from a virtual source of fissile material through a
plurality of cargo containers to a virtual detection system to
facilitate a better understanding of the behavior of emitted
radiation traveling through cargo containers. The results of the
simulation also assist in determining how many detection containers
should be allocated to a ship and where the detection containers
are to be located so as to provide coverage over a desired volume
range across which detection is feasible within the ship.
[0010] Use of this detection system aboard a ship on the open sea
reduces the consequences of a positive detection. Scanning,
detection and intervention can all be conducted far from urban
areas rather than at a coastal port city and while there is still
time to prevent the fissile material or assembled weapon from
reaching shore. Because the system will be able to make a positive
detection long before a ship would reach the destination port,
there will be ample time to turn the ship away and investigate.
With current land-based detection systems, the positive detection
and intervention may simply occur too late. It is quite conceivable
that a terrorist group, if technologically advanced enough to
obtain a nuclear weapon, would be able to detonate it from a remote
location when the ship passes by a particular target (such as
downtown Manhattan). Another possibility is that the terrorists
could have an automatic triggering system that would instruct the
weapon to detonate upon the opening of the container or even upon
receiving an x-ray or neutron interrogation from customs officials.
The land-based detection system at the port would not have even had
a chance to operate.
[0011] Not only does early warning prevent a concealed weapon from
ever becoming a threat to a state that employs this system, but it
also ensures that responders have greater flexibility in terms of
available options for safely containing and neutralizing the
threat. The ship-based mobility of the detection systems also
renders them less susceptible to tampering than they would be if
they were installed at a fixed site (e.g., at a port). Furthermore,
cargo scanning at sea will not create bottlenecks at already
overburdened ports, and positive detection claims need not disrupt
the flow of containers in more than a minimal way. Further still,
the system is deployable without building an impractical
infrastructure that requires the rerouting of containers.
[0012] Additional advantages are offered with the extended
detection times available during the transit of the ship. The
longer detection time allows for an increase in counting time from
the order of minutes to the order of days or even weeks. Employment
of this prolonged scanning period over the length of the ship's
voyage (e.g., at least several days, and often a week or two or
more in transports crossing one or more oceans) reduces error,
enables detection of extremely small radiation emissions over time,
and greatly expands the number of surrounding cartons that can be
reliably scanned. Additionally, sensitivity of the detector is very
high due to the prolonged scanning period and the operation of the
detector at sea, where background radiation is very low, thereby
enabling highly reliable scanning across a large volume stretching
through surrounding cartons.
[0013] Moreover, the detectors can scan passively, thereby limiting
their detectability and removing any potential for harming scanned
cargo. We assume that given the limited number of weapons available
to such an adversary, every conceivable effort will be made to
"booby-trap" the weapon in such a way as to make either active
probing with, e.g., x-rays, neutrons, or mechanical examination
essentially impossible without triggering the device, thereby
producing a nuclear detonation at the port, which would be a
successful outcome for a terrorist organization. The option of
scanning passively, only, removes or greatly reduces this
threat.
[0014] Because of the passive scanning and because the system will
be entirely inside the cargo container, it will also have the
advantage of being able to operate covertly. The detectors can be
concealed in a non-descript container having standard cargo
dimensions of about 20 or about 40 feet in length, a width of about
8 feet, and a height of about 8.5 or about 9.5 feet, thereby
shrouding the presence and/or location of the detection system from
terrorists and, accordingly, rendering the detectors less
susceptible to evasion and tampering by the terrorists. The
detection container, which contains the detection system, will not
be used to transport other cargo, as in "smart containers." Smart
containers represent one industry approach wherein detection
devices are incorporated as a small component of a cargo container
used to ship cargo. The detection device in such a container is
designed to detect materials within that particular container.
Moreover, in the event that a terrorist was trying to smuggle a
weapon inside a container, the terrorist likely would not place the
weapon inside a container in which a detector was also present, or
the terrorist, knowing of the presence of the detector, would work
to disable or otherwise circumvent the detector.
[0015] Because the detection systems, described here, are designed
and configured to detect radiation produced outside the container
(e.g., by orienting the faces of the detector elements to detect
radiation from any direction), a limited number of detection
systems (i.e., as few as one) aboard a ship can be used to monitor
a large volume of the cargo area aboard the ship (i.e., covering
many cargo containers aboard the ship). The detection systems need
not (and, in preferred operations, would not) be placed in every
container aboard the ship.
[0016] Additional advantages can be gained by using large area
detectors. With multiple detectors in an array, the system can
generate an image of detected gamma radiation. Use of the array to
image the radiation will enable the system to readily distinguish
the specific activity of a weapon from the specific activity of
naturally occurring radioactive materials, such as a mass block of
marble or granite (two materials having a high thorium content).
Large areas of saturated activity on the image will correspond to
the marble or granite, while small "point-like" areas of intense
activity will correspond to the fissile material. Further, fissile
material in a weapon will still have a much higher specific
activity than the marble or granite. Point-like objects are
especially well imaged by coded apertures and thus provide a means
for spatially locating threats.
[0017] It is important to note that even should the system prove to
only detect a fraction of events, a certain amount of "deterrence
by denial" (i.e., where the enemy realizes that its likelihood of
success is low, and failure is viewed as unacceptable) would be
achieved. If it is widely known that the detection systems are
being shipped along with other cargo aboard containerships without
terrorists having the ability to determine where and when the
detection system are in use, then the likelihood of this smuggling
route being employed would be diminished. It is no easy task for a
terrorist group to obtain a nuclear weapon, so it is unlikely that
they would risk sending it on a protected route, even if that route
is not 100 percent protected. If only a fraction of events can be
shown to be detectable, the goal of shielding this particular
smuggling route will likely be achieved through deterrence by
denial.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] In the accompanying drawings, described below, like
reference characters refer to the same or similar parts throughout
the different views. The drawings are not necessarily to scale,
emphasis instead being placed upon illustrating particular
principles of the methods and apparatus characterized in the
Detailed Description.
[0019] FIG. 1 is a sectional view of a gamma detector array mounted
in a cargo container.
[0020] FIG. 2 is a perspective view of the gamma detector
array.
[0021] FIG. 3 is a schematic, overhead view of detection system
components in a cargo container.
[0022] FIG. 4 is a graph showing the spectrum of gamma radiation
(as simulated by MCNP modeling) exiting the core of a weapon
containing 12 kg of 30-year-old uranium with 100 parts per trillion
.sup.232U.
DETAILED DESCRIPTION
[0023] The following is a description of a proposed system for
detecting concealed nuclear weapons aboard cargo containerships.
The distinction that will give this system an advantage over other
systems in use and in development is that the detectors will be
aboard the containership. The system operates passively (i.e., it
simply detects radiation that is emitted from surrounding sources
and that impacts on the detector--rather than actively directing
radiation into containers to stimulate emission). The entire
detection system can be confined to one standard container having
dimensions, e.g., of 40.times.8.5.times.8 ft.sup.3 (about
12.times.2.6.times.2.4 m.sup.3), which is shipped amid the other
cargo containers containing conventional cargo aboard the ship. The
exterior of the container in which the detection system is
contained is designed to be non-descript (i.e., having not visible
indicia to indicate the presence of a detection system inside).
[0024] The systems described herein are not components of
containers that are used to transport other shipping cargo.
Further, the systems of this disclosure are not contained in all or
most of the containers aboard a ship. Rather, the detection systems
can be incorporated in only a minority of containers devoted
exclusively to detection and that are otherwise empty (with the
detection system potentially filling a substantial or majority
portion of the container). Each of the other containers aboard the
ship (i.e., the majority of containers) are conventional shipping
containers that are filled with cargo and that do not include
radiation detectors. Further still, each detection system is
configured not to measure radiation originating from within the
container in which the detection system is contained but rather
from outside the container (i.e., from a plurality of other
containers within the ship); specifically the detection system is
programmed to look for likely radiation signatures that would come
from other containers passing through container walls and other
contents of surrounding containers, as derived from Monte Carlo
N-Particle Transport Code (MCNP) modeling.
[0025] A section view showing a gamma detector 10 in a cargo
container 12 is provided in FIG. 1. The gamma detector 10 is
mounted and secured within the container 12 on a support structure
16, which can include shock absorbers 18, e.g., in the form of
springs. In this embodiment, the gamma detector 10 includes an
array of crystals 14 (e.g., 10-cm.times.10-cm.times.10-cm cesium
iodide crystals activated with thallium) coupled with a
photomultiplier tube inside the detector 10. Cesium iodide (CsI) is
particularly suitable as a detector crystal because of its
relatively high efficiency and energy resolution and because of its
resistance to thermal and mechanical shock. Cesium iodide crystals
are available, e.g., from Saint-Gobain Crystals (Solon, Ohio, USA
and Nemours, France).
[0026] The gamma detectors can also use sodium iodide (NaI)
detector crystals, which are standard for gamma detection where
cooling is not possible, though sodium iodide is particularly
susceptible to both mechanical and thermal shock. It is common
knowledge that while loading a ship, crane operators tend to drop
containers from time to time. The mechanical shock of such an event
could cause the sodium iodide crystals to crack. Additionally, the
possibility of large temperature swings might damage a sodium
iodide crystal due to thermal shock; sodium-iodide crystal
manufacturers indicate that a common crystal will not withstand a
change of more than 15 degrees Fahrenheit (8.3.degree. C.) per
hour. Sodium iodide crystals also are available from Saint-Gobain
Crystals.
[0027] Alternatively, the detector can include other scintillating
crystals, or a liquid or plastic scintillator. Given the large
volume of a standard 40-ft. (.about.12-m) container, there is
plenty of room for large tanks of liquid scintillator or blocks of
plastic scintillator that are sufficiently large to ensure a high
efficiency of 2615 keV gamma capture. In other embodiments, the
gamma detector is a gas-filled detector, such as a high-pressure
xenon tank. In still another embodiment, the detector includes
germanium, and a cooling unit for the germanium is also included in
the detection container. Also, other materials, such as cadmium
zinc telluride (CZT), which do not require cooling, may be
employed.
[0028] Each array of scintillator crystals 14 has a detecting
surface with dimensions, e.g., of about 1-m.times.1-m, though the
arrays can be made larger or smaller, as desired. The crystal
arrays of the detector, in the aggregate, extend in all three
dimensions, as shown by the three faces, oriented orthogonally to
on another, of the detector 10 illustrated in FIG. 2. This
configuration is in contrast with the common planar imaging
scanners seen at ports or border crossings. The weapon could be in
any direction relative to the detector, and a planar array only
"looks" in one dimension efficiently. Three detector arrangements
are provided in this section, but the ultimate configuration will
depend on the cost/benefit analysis made by the government and
vendor.
[0029] The first, and perhaps most efficient, configuration for the
gamma detector is a sphere of outward facing detector arrays.
However, another constraint is the desirability of using a
coded-aperture mask, the development of which has thus far targeted
flat-surface reconstruction (and not reconstruction on a spherical
surface).
[0030] In another embodiment, the detector arrays are configured in
a cube, with a coded-aperture mask in front of each face. The use
of coded-aperture masks and the processing of images produced
therewith are described, e.g., in U.S. Pat. No. 5,930,314; U.S.
Pat. No. 6,737,652; and U.S. Pat. No. 6,205,195; each of which is
incorporated by reference herein in its entirety. In short, the
concept of coded aperture imaging is partly based on that of a
single pinhole camera. In one example of a pinhole camera system, a
sheet with a single pinhole is placed between a detector array and
a source. Thus, when an individual detector in the detector array
detects a photon, a processor can determine from which direction
the photon came based on the location of the pinhole with respect
to the individual detector.
[0031] To enable more photons to reach the detector (compared with
a pinhole camera), the coded-aperture mask includes multiple
specially arranged apertures. Known aperture patterns used for
coded-aperture masks include a random array, a Fresnel zone plate,
and a uniformly redundant array. Coded-aperture techniques are
different from conventional planar imaging methods in that the
detected signal is not a directly recognizable image. The signal is
encoded and must be decoded before a visible image can be obtained.
Coded-aperture methods include two processes: coding and decoding.
First, information about the object being imaged is coded in the
detected signal; second, the detected signal can be decoded to form
a three-dimensional image of the object. Decoding of the recorded
signal can be performed as a correlation or deconvolution operation
in the space domain, or a Fourier transform and filtering
(multiplication) operation in the reciprocal (frequency) domain
followed by an inverse Fourier transform. Other transforms, such as
a Hadamard transform, can be used instead of the Fourier
transform.
[0032] While probably the most practical, configuration the
detector crystals into the form of a cube may not be the most
efficient design. A more efficient design may be an embodiment
wherein the detectors are arranged in a half-pyramid with three
planar arrays, each orthogonally oriented relative to the other
planes, which would still enable detection of radiation approaching
from any direction (e.g., from above, from below, or laterally from
any side). If a half-pyramid having 1-m edges is made of 10-cm
crystals, and if crystals at the edges are shared between adjoining
faces, then the half-pyramid can include 271 detector crystals.
Coded apertures can still surround the pyramid, but fewer detectors
are used--in comparison with a full cube shape.
[0033] In yet another alternative embodiment, the detector array is
not a separate free-stranding structure inside the container;
rather, the container, itself, serves as a substrate for detector
panels that are painted onto or adhered to inner surfaces of the
cargo container.
[0034] Using an iterative reconstruction approach, a single
detector array can obtain three-dimensional information without
motion. The detectors are placed in such an array so that the
system can distinguish between a distributed natural background and
a concentrated point source. Use of the coded-aperture mask also
reduces background, minimizes false positives and improves
detectability. On a basic level, the coded-aperture mask is
essentially a partially opened mask that allows some photons
through, while blocking others. The pattern of photons that are
allowed to pass can be used through detailed reconstruction
algorithms to reduce background, thus enhancing point-like or
relatively point-like images.
[0035] More details on the use of coded apertures and associated
electronics for radiation imaging is provided in R. Accorsi, et
al., "A Coded Aperture for High-Resolution Nuclear Medicine Planar
Imaging with a Conventional Anger Camera: Experimental Results," 48
IEEE Transactions on Nuclear Science 6, p. 2412 (2001); and in K.
P. Ziock et al., "Large Area Imaging Detector for Long-Range
Passive Detection of Fissile Material," 51 IEEE Transactions on
Nuclear Science 5, p. 2238-2244 (2004); both of these articles are
incorporated herein by reference in their entirety.
[0036] Essentially, the radiation source projects only one shadow
on the detector array, while the distributed background projects
many shadows on the array. The reconstruction process takes into
account different projections and significantly reduces background,
leaving mostly the source in the image. In this way, background
from the ocean, ship and cargo can be reduced to increase
confidence in detectability and reduce false positives.
[0037] Additionally, a neutron detector (available, e.g., from Can
berra Industries of Meridien, Conn., USA, or from LND, Inc. of
Oceanside, N.Y., USA) can be included in the system to detect
spontaneous fission neutrons from .sup.238Pu and .sup.240Pu, which
can be particularly useful for monitoring for plutonium because
weapons-grade plutonium offers a comparatively weak direct
gamma-ray signal, though secondary gamma rays can be produced (and
detected) due to the interaction of neutrons from plutonium with
the surrounding material. Also detectable are high-energy gammas
from fission product decay. The neutron detector can include a
single detection body (rather than an array), and it need not
measure the spatial distribution of emitted neutrons. The neutron
detector is electronically coupled with a shared data processor and
transmission system, with which the gamma detector is also
coupled.
[0038] A suitable neutron detector is a standard .sup.3He tube
commercially available through many vendors. A .sup.3He detector
can provide high detection efficiency. It is not necessary to
choose a neutron detector with an attached moderator because
neutrons will be well thermalized by the time they traverse the
weapon (especially the high-explosives) and intermittent cargo.
Alternatively, the neutron detector can be a common BF.sub.3
proportional counter.
[0039] In one embodiment, the detection system includes an array of
plastic or liquid scintillators that detects both neutrons and
gamma radiation (rather than using separate neutron and gamma
detectors). Also, a single large plastic detector or tank of liquid
Scintillator may be used.
[0040] A schematic top view of the contents of the detection
container 12 is provided in FIG. 3. The gamma detector 10 and the
neutron detector 20 detect gamma radiation and neutrons,
respectively. Both are powered by a power source 22 (e.g., a
battery or a generator) within the container 12. The power source
22 also powers signal-processing electronics 24, a computer 26, a
transmitter 28, a local transponder 30, and a receiver 32 for
remote access. There is no cargo in the container 12, only the
elements of the detection system.
[0041] The computer 26 (e.g., a personal computer running a
Windows.TM. operating system) receives data from a multi-channel
signal processor 24 and processes the information. Data processing
includes not only normal background-reduction algorithms, but also
reconstruction of the coded-aperture image. The computer 26 is
directly linked to the transmitter 28 so that the data can be
sent.
[0042] Suitable software programs that can be utilized to process
the radiation measurements include the GammaVision.TM.-32 V6
spectroscopy software program from Ortec of Oak Ridge, Tenn., USA;
the InterWinner.TM. 5 spectroscopy software program, which is also
from Ortec; and the FRAM software code developed at Los Alamos
National Laboratory and licensed for commercial distribution to Can
berra Industries of Meridien, Conn., USA.
[0043] The transmitter 28 sends data back to a remote, land-based
location for analysis and possible dissemination to individuals who
are responsible for emergency response/intervention. Transmission
can occur through direct broadcast to the remote location or, more
likely, via a satellite link. One option for minimizing the number
of signals bounced off satellites is to provide the containerized
detection systems with a local transponder 30 to enable the
detection systems on board the ship to communicate with each other
wirelessly and to provide at least one master unit or an
independent communications hub on each ship with a transmitter for
broadcasting the signal to the satellite. Enabling the detection
systems to communicate with each other also enables them to work
cooperatively, for example, if one detection system makes a
positive detection, the detectors in other detection systems can be
pivoted (e.g., when mounted on a drive shaft coupled with a rotary
motor) to face in the direction of the suspected source of
radiation.
[0044] Alternatively, the wireless network can be coupled with the
ship's radio system so that the data from the transmitter in each
detection container can be transmitted from the container to the
hub and then through the ship's communication system to a remote
computer monitoring system, which can provide simultaneous global
monitoring of ships and trigger an alarm whenever a positive
detection is made by one of the detection systems. Alternatively,
the data can be transmitted to the remote computer monitoring
system in a more raw form, where more of the data processing is
carried out by the remote computer monitoring system.
[0045] Each containerized detection system can also include a
receiver 32 that can accept instructions from, e.g., the remote
computer monitoring system to adjust its operations or to provide
requested data. The detection containers can also be packed with
insulation, particularly around the detector crystals to prevent
the crystals from cracking due to large temperature gradients or at
low temperatures, as may be encountered, e.g., by ice-breaker ships
traversing near the arctic circle or near Antarctica.
[0046] The non-descript nature of the containerized ship-based
detection systems allows these units to operate covertly.
Nevertheless, as an additional countermeasure just in case the
detector's location is discovered, the container can also be
secured with an alarm system that will signal a remote monitoring
center that the container has been opened. This alarm condition can
also be initiated if the unit suddenly stops sending data.
[0047] In some embodiments, an active system is deployed alongside
the passive detection system in the detection container. The active
system lies dormant until a positive detection is made by the
passive detectors and then interrogates surrounding cargo
containers by emitting radiation in the direction of the suspected
radiation source. The active system can emit neutrons and/or
photons, and the passive detection system will then detect a return
signal generated by the interaction of those neutrons and/or
photons with fissile material, if present. The radiation can be
emitted in pulses so that the detector system is not swamped with
the active system's emitted radiation. Further still, the active
system can be mounted for rotation, e.g., on a ball-joint, so as to
enable focusing a beam of emitted radiation from the active system
in a suspected direction from which the radiation has been
detected. The active system can also be equipped with a receiver so
that it can be activated upon a positive detection by the passive
system and so that it can receive instructions for directing its
emitted radiation. Alternatively, active interrogation of cargo can
be provided by unmanned aerial vehicle (UAV) or helicopter flights
outfitted with accelerators that can actively interrogate the
suspected area of the ship with either neutrons or photons. The
active system can also incorporate a precision time base or one
derived from a global positioning satellite (GPS) to "time stamp"
each event (e.g., each pulse of radiation emitted by the active
system and each detection of radiation, i.e., the "return" signal).
This time stamp can be used to enhance background rejection, since
the operator or control system will know when the interrogating
signal was sent and will have an approximate expectation as to when
to expect the return signal (particularly, by looking for a spike
in detector readings) if fissile material is present.
[0048] Having explored the contents of the detection container, the
citing of these detection containers amid the cargo containers on a
ship is discussed, below. The long count time during a typical
two-week voyage allows sufficient radiation to be transmitted from
a weapon, through cargo containers, and to the detectors in the
detection container to enable detection of weapons-grade uranium
and plutonium in implosion-type configurations with three-sigma
confidence from distances averaging 22.0 and 23.5 meters of cargo
respectively.
[0049] In particular embodiments, between 3 and 20 detection
containers are deployed on each ship, depending on the ship's cargo
capacity and the degree of control exercised over container
placement. The ratio of detection containers to cargo containers
that do not include detection systems aboard the ship can be, e.g.,
in the range of 1:100 to 1:650. The detection containers can be
loaded in a "centerline" pattern (i.e., along the centerline of the
stacked cargo containers extending through the length of the ship)
to target optimal coverage volume; alternatively, the detection
containers can be loaded in a random pattern where no control is
assumed for placement of the containerized detectors.
[0050] There are two deployment objectives for attaining a desired
level of deterrence by denial. First, the enemy should be aware
that containerized detectors exist and are operational; and second,
the enemy should know that he cannot locate a significant
percentage of the detectors. Given the desirability of covert
deployment and necessity of visible deployment, some percentage of
containerized detectors can operate covertly to ensure the element
of fear, and some percentage are visible to ensure the credibility
of the deterrent. Accordingly, a combination of centerline and
random deployments can be used to achieve both objectives.
[0051] Another deployment enhancement that may increase deterrence
is the use of dummy detection containers so as to thwart an enemy
conducting surveillance of the ship-based system in hopes of
uncovering loading and shipping patterns. The dummy detectors can
be sent from the same holding pen as the actual detection
containers. These dummy detectors could consist of a blatantly
transmitting device that would suggest to the adversary that they
are real operating systems, though the dummy detection containers
would not contain the actual detectors.
[0052] With this containership system, the detection times will
only depend on the amount of time that the cargo is en-route. This
is usually on the order of days and possibly weeks and will allow
sufficient time to resolve even a very weak signal. Adding to the
significance of the long detection times is the fact that detection
systems can be located further from the source.
[0053] Even with high signal attenuation in dense materials, the
long count times over the course of an oceanic voyage (often on the
order of two weeks) as well as the existence of random pathways of
air through cargo allow transmission of even weak radiation signals
through several cargo containers.
[0054] Finding fissile material is a difficult task not only
because most gamma radiation from a weapon is emitted at low
energies and is therefore easily shielded but also because neutron
and gamma emission from highly fissile isotopes, .sup.235U and
.sup.239Pu, is almost non-existent. Ironically, it is the
methodology necessary to create materials of mass destruction that
introduces a detectable signature. The enrichment process
preferentially introduces the impurity .sup.232U in reprocessed
weapons grade uranium and the creation of plutonium in a reactor
produces significant quantities of .sup.240Pu, both of which are
not found in nature, but are highly detectable.
[0055] To better understand how the radiation from a fissile
material would penetrate through cargo containers and their
contents, computer simulations were carried out using the MCNP-5
code (developed at Los Alamos National Laboratory). The use of MCNP
in simulations is described in the standard reference, J. F.
Briesmeister, Ed., "MCNP--A General Monte Carlo N-Particle
Transport Code," LA-13709-M, which is incorporated herein by
reference in its entirety. Specifically, the MCNP code was used to
simulate the radiation emitted from a nuclear weapon, wherein the
contents of surrounding cargo containers were simulated by
subdividing the volume of the containers into pixels (in this case,
pixels that were 1.0 m.times.1.0 m by 1.5 m) with the longer pixel
dimension extending along a line between the detector and the
weapon, and randomly assigning densities to the pixels based on
known volumes/frequencies of shipped cargo contents and the
densities of those contents, including the air content in
containers. Calculations (based on known shipping quantities of
cargo content, content densities, and container volumes) indicate
that the container volume includes, on average, about 76% air.
[0056] We assume that weapons-grade uranium or weapons-grade
plutonium will be in the weapon; accordingly, simulations were run
with 50 kg of weapons-grade uranium and 12 kg of weapons-grade
plutonium. For models including weapons-grade uranium, the
simulated shielding was an encompassing sphere of lead 2-cm thick.
For neutrons, two shields were considered, one 5-cm sphere of water
equivalent and one 20-cm sphere of polyethylene.
[0057] The graph presented in FIG. 4 is the spectrum of gamma
radiation (as simulated by MCNP modeling) coming out of the core of
a weapon containing 12 kg of 30-year-old uranium with 100 parts per
trillion .sup.232U. Of great interest are peaks with high energy,
which will have better transport and lower background. Several
major peaks have been identified, but the large difference in
counts and background of the 2615 keV peak (from the daughter
product, .sup.208T1) is clearly very attractive for detecting
uranium. The .sup.212Bi peaks are of interest because of their high
energy and low background; however, they are all orders of
magnitude less intense than the 2615 keV peak.
[0058] In the MCNP simulation of gamma radiation exiting the high
explosive and passing through adjacent containers and their cargo,
on average, only around 1 in 10.sup.12 gamma particles reach the
detector at 22+ meters. Nevertheless, a two-week exposure results
in a flux of 10,000 to 100,000 gamma particles reaching the
detector, thereby enabling identification of the 2615 keV peak at
distances up to 22 meters or further (depending on the content of
intervening cargo containers) from the source.
[0059] Notwithstanding the reduced transport efficiency at lower
energies, these simulations suggest that source verification using
the 1001 keV line (from the .sup.238U daughter product,
.sup.234mPa) remains practical through at least 13 meters.
[0060] In MCNP simulations (with 2 cm of lead shielding a source
with 12 kg of 30-year-old uranium having 100 parts per trillion
232U), the effect of lead shielding around the uranium was found to
be surprisingly minimal, but not insignificant. There was still an
above threshold flux at more than 20 meters and the average
distance to threshold was about 19.5 meters. Even with a
20-cm-thick sphere of polyethylene (or other low-density
hydrogenous material) surrounding the weapon, the ship-based system
can detect neutrons penetrating through almost 20 meters of cargo
on average.
[0061] In contrast with the above-described method of looking for
uranium, the gamma radiation signature of plutonium is not strong.
Plutonium does not, by itself or via any of its daughters, give off
any high-energy, high intensity gammas. Of note, however, is the
.sup.241Am daughter of .sup.241Pu, which emits a 662 keV gamma with
relatively high intensity. This line will be indistinguishable from
the .sup.137Cs line possibly found in benign medical isotopes
within normal cargo so that, by itself, the 662 keV line is not
enough to assume a positive detection of weapons grade
plutonium.
[0062] Fortunately, plutonium can be detected by directly looking
for neutrons or by inferring their presence from unique secondary
reactions. Neutron interactions including radiative capture and
inelastic scatter will produce high-energy gammas that can be
detected. Also, fission product decay produces detectable
high-energy gammas. There are virtually no sources of neutrons in
common cargo and the background (from the "ship-effect") is
expected to be fairly constant.
[0063] Accordingly, the presence of weapons-grade plutonium can be
detected by looking for neutrons. The signal from spontaneous
fission neutrons in weapons-grade plutonium is orders of magnitude
stronger than that of uranium. The strength of the signal is highly
dependent on the amount of .sup.238Pu and .sup.240Pu in the fissile
material. The spontaneous fission half-life of .sup.238Pu is
5.0.times.10.sup.10 years, while that of .sup.240Pu is
1.2.times.10.sup.11 years, giving 4.39.times.10.sup.-19 and
1.83.times.10.sup.-19 neutron emissions per second per atom,
respectively. .sup.239Pu spontaneous fission neutron intensity is
of the same order as .sup.238U and therefore insignificant. When
combined in the assumed weight percentages for the weapon model and
accounting for (.alpha., n) reactions, 56,000 neutrons per kilogram
per second are produced. Furthermore, significant multiplication,
where spontaneous fission and (.alpha., n) neutrons cause normal
fission, can occur for larger quantities of plutonium. For 4
kilograms of weapons-grade plutonium, the number of neutrons
leaving the core surface per kilogram per second turns out to be
around 110,000, which is readily identifiable over the natural
background.
[0064] Neutrons may also undergo inelastic (n, n' .gamma.)
collisions while still slowing down in the weapon (and possibly in
the cargo), which can produce high-energy gammas. While a
considerably lower flux is expected without active interrogation,
the number of gamma photons having an energy of 3 MeV or greater
from fission fragment decay will not be negligible, especially when
contrasted with the essentially zero background at these
energies.
[0065] Neutrons that escape the weapon will interact with the
surrounding cargo. Cargo could have almost any isotopic
composition, but much of it is organic. This will allow for further
interaction with hydrogen, oxygen and nitrogen, producing more
high-energy gammas. If the weapon happens to be situated in the
center of the containership, the cargo will likely capture a
significant portion of the neutron flux. Even if situated near the
edge, high percentages of the total neutron flux will be captured
before leaving the ship. The neutron flux will be well thermalized
by the time it leaves the weapon, but there still exists the
possibility of (n, n' .gamma.) interactions in the cargo. The
detection of a flux of gammas from 2-11 MeV that is significantly
higher than background, would signal the presence of a large
neutron flux and therefore fissile material.
[0066] It is also possible that a large flux of neutrons from a
cosmic event, such as a solar flare, would cause a benign increase
in high-energy gammas and neutrons, but this would be an impulse
event and could easily be distinguished from the constant flux of
high-energy gammas from spontaneous fission neutron capture.
Moreover, a solar event would be simultaneously detected by other
deployed units and various scientific endeavors throughout the
world and could therefore be recognized (and discounted) as
such.
[0067] Inherent to almost all land-based detection systems is the
interference of natural background radiation. Uranium and
especially thorium in the ground and in building materials offer an
extremely competitive signal. The problem lies mainly in the decay
chain of .sup.232Th, which includes the same radioactive daughters
(e.g., .sup.208T1) as the uranium-based nuclear weapon, thereby
producing the characteristic 2.6 MeV gamma-radiation signal,
discussed above. Natural thorium concentrations vary from place to
place and will give a strong enough signal to significantly reduce
the confidence of a positive detection event. The advantage that
this seaborne approach has over the land-based systems is that
there is virtually no thorium in the ocean water. One study gives
thorium concentrations at 16 parts per trillion in ocean water and
an average 1 part per million on land (the concentration can be
over 100 parts per million in granite). Accordingly, moving from a
land-based procedure to a sea-based procedure provides an advantage
of reduction of natural background of 5 to 7 orders of
magnitude.
[0068] Both granite and marble are high in thorium and will
therefore interfere with the weapon's discrete gamma spectrum.
Nevertheless, the presence of a large mass of granite or marble
aboard the ship need not be a source of false positive detections
because the specific activity of the weapon will still be much
higher than the specific activity of the granite or marble. Large
areas of saturated activity on the image will correspond to the
marble or granite and small areas of intense activity will
correspond to the fissile material, and the presence of a weapon
can thereby be distinguished from those other sources via the
practice of imaging using a large area detector array.
[0069] Further still, the following procedure can be employed to
distinguish benign cargo and fissile material, taking advantage of
the fact that actinium gamma lines will be present in the spectrum
of natural thorium but not in the spectrum of the weapon. The
absence of actinium lines in the weapon can be used to distinguish
the thorium's 2615 keV contribution to the measurement from the
weapon's contribution. Armed with knowledge of the nearly exact
relative decay intensities of .sup.228Ac and .sup.236Pu/.sup.232U
daughter lines, the contribution of daughter isotopes, such as
.sup.208T1 from the natural background, can be essentially
subtracted from the total measurement. Once the background has been
removed, a more accurate representation of the weapon can be made
and potential false alarms can be minimized.
[0070] There is no significant contribution of 2615 keV and 1001
keV background from the ocean water, even with a large volume taken
into consideration. Likewise, the various material constituents of
the ship produce negligible 2615 keV and 1001 keV gamma
radiation.
[0071] The most concentrated source of thorium and uranium in cargo
comes from cargo made of rock and, in particular, granite and
marble. While almost all forms of commercial granite and marble
have some concentration of thorium and uranium, a few relatively
rare types of granite and marble have high concentrations of
thorium, especially those originating from Brazil and India, where
huge deposits exist; however, most granites and marbles contain
very little thorium and uranium.
[0072] Per implementation of U.S. Customs Office policy, all
shippers are required to report the contents of their sealed
containers at least 24 hours prior to loading at the foreign port.
This information could be used as a way to pre-empt false alarms by
simply cross-referencing the containership's manifesto. Any major
shipments of granite, while still unlikely to cause a false alarm,
would be known in advance and would therefore be used to adjust
expected backgrounds.
[0073] In much the same way as the 1001 keV line can be used to
confirm the presence of fissile material relative to benign
granite, the presence of a 911 keV line can be used to confirm
benign quantities of granite relative to fissile material. Simply
put, a measurement of 2615 keV gammas accompanied by 1001 keV
gammas indicates the presence of fissile material, while a
measurement of 2615 keV gammas accompanied by 911 keV gammas
indicates the presence of natural thorium (via the signal from its
decay product .sup.228Ac) in granites and marbles.
[0074] Based on worst-case estimates wherein the cargo includes
1000 tons of Cafe Brown granite in the detector's field of view,
conservative estimates (not accounting for spectroscopic source
reduction) put the number of 2615 keV gammas detected over a
two-week voyage at 5.3.times.10.sup.6 per square meter.
[0075] Once the ship-based system is deployed, a database of
typical backgrounds (i.e., the backgrounds that interfere with the
detection of fissile material, specifically, gamma background at
energies of 2615 keV and 1001 keV) aboard containerships can be
established by logging those readings. Most measurements will be
benign, thereby at minimum producing a good characterization of the
background. These background spectra can be fed back into a central
database for constant cross-reference and used in background
subtraction algorithms. The longer the system is deployed, the more
valuable the database will become.
[0076] Furthermore, after an extensive search, including hundreds
of possible medical and industrial isotopes, it appears safe to
conclude that no other naturally occurring isotopes or expected
cargo materials, beyond thorium and uranium, are likely to release
radiation that directly interferes with the 2615 keV and 1001 keV
lines.
[0077] Interfering background signals for the neutrons can also be
countered. Neutrons from terrestrial sources are almost
non-existent. The vast majority of neutron background is induced by
cosmic events in the atmosphere. Cosmic ray muon interactions in
the upper atmosphere cause a cascade of particles, a percentage of
which are neutrons. Cosmic neutrons can be detected directly or can
interact with other nuclei in spallation events. Due to spallation,
an increase in the neutron population near dense, high-mass-number
objects is commonly seen and several groups have measured this
phenomenon.
[0078] As a result of interest in the increased flux around
high-atomic-number materials, the ship-effect has been studied
extensively. A detailed survey of ship-effect literature has
produced a fairly comprehensive measurement of neutron populations
on sea-going vessels. Most of these measurements were made on U.S.
Navy ships and can be considered good approximations as to what
might be seen on a containership.
[0079] Between 10 million and 1 billion neutrons can be expected to
be detected due to natural background over a two-week voyage.
Measured fluxes onboard ships suggest an estimated detection of
1.54.times.10.sup.8 neutrons per square meter per two weeks from
background. This number can used as benchmark for neutron
detectability calculations.
[0080] Upon a positive detection, a ship or plane can be sent out
to interdict and attempt to verify the positive detection, e.g.,
via active scanning of suspected containers with neutrons or
photons. The ship can then be redirected, e.g., to an isolated
location, or the cargo removed to prevent the fissile material from
reaching the intended port.
[0081] In describing embodiments of the invention, specific
terminology is used for the sake of clarity. For purposes of
description, each specific term is intended to at least include all
technical and functional equivalents that operate in a similar
manner to accomplish a similar purpose. Additionally, in some
instances where a particular embodiment of the invention includes a
plurality of system elements or method steps, those elements or
steps may be replaced with a single element or step; likewise, a
single element or step may be replaced with a plurality of elements
or steps that serve the same purpose. Moreover, while this
invention has been shown and described with references to
particular embodiments thereof, those skilled in the art will
understand that various other changes in form and details may be
made therein without departing from the scope of the invention.
[0082] For example, although the above discussion has focused on
the use of detection containers aboard sea-going cargo ships.
Detection containers can also be used in other cargo transport
vehicles, such as in air planes, trains, trucks, cars, and oil
tankers. The containers can be smaller in these other contexts
where smaller containers (e.g., a container resembling a large
express-delivery package) would more closely resemble other cargo
containers aboard the particular vehicle type. In still other
embodiments, the "containers" can be the body of a car or other
vehicle type; this embodiment is particularly advantageous for a
cargo ship loaded with vehicles. The detection container
accordingly would appear to be an ordinary vehicle
indistinguishable from others aboard the ship; the detection system
can be mounted, e.g., inside the trunk of the vehicle body.
* * * * *